It has been established that activity of all muscle is initiated by the liberation of Ca, but work on mechanics and energetics of muscles indicates that activity is also regulated internally by responses to length changes and that indeed by far the largest part of energy released is induced by this control, which does not required changes in free Ca. This work will be continued. Preliminary work has indicated that this control is unexpectedly complex, that length changes induce under some conditions two effects in opposite direction and with very different duration. The nature and significance of these effects will be studied not only in isolated muscles, but also in muscle with intact circulation. The results obtained so far are of great generality. Only quantitative differences were found between skeletal, cardiac and insect flight muscles. This work reveals subtle aspects of the function of muscles and suggests new approaches to the action of drugs, particularly those acting on the heart.